Line edge roughness (LER) and line width roughness (LWR) refer to the non-smoothness of edges of features printed using lithographical methods or features that have been transferred by different methods, e.g. etching, from such features. As the line width shrinks with advanced technology nodes the relative thickness of LER increases as it becomes a larger fraction of the total line width.
There are several methods for LER measurement, including e.g. SEM images (CD-SEM), AFM scans and various optical methods, e.g. angle-dependent spectral scatterometry, angle-dependent laser scatterometry, spectral ellipsometry, however all measurements suffer from the fact that LER effect has very small values and is stochastic by nature.